This invention relates to a charging apparatus of a shaft furnace of the type having a rotary or oscillating chute charge distribution device and a storage housing or hopper mounted on the vertical axis of the shaft furnace. More particularly, this invention relates to a new and improved device for distributing charge material in the storage housing toward the rotary chute which selectively increases or decreases the discharge cross-section from the storage hopper symmetrically about the vertical axis of the furnace.
Patent document EP-A No. 0062770 corresponding to U.S. Pat. No. 4,514,129, assigned to the assignee hereof, all of the contents of which are incorporated herein by reference, discloses a central feed charging installation wherein the storage hopper discharges the charging material onto a chute which is arranged on the axis of the furnace. It will be appreciated that the charging installations recently constructed in accordance with the teachings of U.S. Pat. No. 4,514,129 have proven that the apparatus described therein solves the well known problems presented by oblique falls of charging material in those prior art charge installations incorporating two alternately operating storage hoppers placed next to one another.
Although the charging installation of U.S. Pat. No. 4,514,129 has made it possible to solve a problem which has been known since the existence of rotary chute charging installations, it nevertheless presents another problem discovered some time ago and attributable to the granulometry of the charging material. It will be appreciated by those skilled in the art that the charging material (which usually consists of particles of iron ore or particles of coke), has a variable and non-uniform granulometry. It has been found that during the filling of the locks and the storage hoppers, the charging material is segregated and therefore collects in configurations which are in accordance with this granulometry. Moreover, this segregation phenomenon is intensified as a result of the discharge.
The segregation results from several factors having cumulative effect. One of the reasons for the segregation is that, during the filling of a storage housing, a natural settling cone forms about the falling point. The largest and heaviest particles tend to tumble down the slopes of this cone under the influence of their own weight towards the peripheral regions of the housing. In contrast, the smallest particles (known as "smalls"), tend to remain in the central region of the settling cone.
Although a natural settling cone forms during filling, in contrast, during the discharge, an opposite phenomenon occurs. In the case of discharge, the particles in the central region tend to flow off first and penetrate to a greater extent to form a V-shaped discharge level.
In addition to this filling and discharge phenomenon, "smalls" tend to accumulate in larger proportions in the bottom of the housing. This accumulation is a result of the size of the "smalls" whereby the "smalls" will slide between the more bulky particles and thereby accumulate on the bottom of the housing.
A third phenomenon is that, when the charge material falls into a housing (especially at the start of the filling phase), a certain number of larger particles break into several parts to form "smalls" in that way.
The cumulative effect of all these phenonema is that, during the initial phase of the discharge of the charging material from the housing, the proportion of "smalls" is much higher than it is towards the end of the discharge, when the proportion of more bulky particles becomes greater. As a result, if the contents of the housing is used to deposit a layer over the entire upper charging surface of the furnace, and if a spiral or concentric circles are described for this purpose from the outside towards the center region by means of the rotary chute, the concentration of "smalls" is much higher in the peripheral regions than in the central region about the vertical axis of the furnace. Unfortunately, this arrangement or distribution of charge material does not always satisfy the requirements of the iron and steel industry.
Although the consequences of this segregation phenomenon remain within acceptable limits in installations with two alternately operating hoppers placed next to one another, they have a greater effect in high-capacity installations of the type described above, with a large central hopper having a stand-by hopper located above it.
However, because it is not desirable to significantly increase the hopper height, the capacity thereof must necessarily be increased by increasing the diameter of the hopper. It will be appreciated by one of ordinary skill in the art that an increase in the diameter intensifies the segregation effects discussed above; so that the undesired consequences thereof become more and more harmful in proportion to the increase in volume of the furnace on which the installation is mounted.